A heating coil for induction heating of an inner surface of a tubular workpiece typically includes a head portion configured to be inserted into the workpiece to inductively heat the inner surface of the workpiece and a pair of lead portions connected to one end of the head portion and the other end of the head portion respectively.
The head portion and the lead portions are formed by using pipe members, forming a series of flow channels through which coolant flows. According to related art heating coils, a head portion and lead portions are formed by using same pipe members (for example, see JP 2001-172716 A and JP 2013-170287 A).
The frequency of power supplied to a heating coil has a proper range which varies depending on the dimension of a workpiece, heating specifications, and the like. However, when various workpieces are heated with various heating specifications using a single equipment, the heating may sometimes have to be performed at a frequency lower than the proper range corresponding to the dimension of a workpiece or the heating specifications.
In induction heating of an inner surface of a tubular workpiece, there is a tendency that heating efficiency becomes lower as the frequency of AC power supplied to the heating coil becomes lower.
When power supplied to the heating coil increases to compensate for the lowering in heating efficiency, an amount of heat generated from the heating coil also increases. The heating coil is cooled using coolant flowing therein, but the flow rate of the coolant is limited, for example, by the shape of the flow channel inside the lead portions, and thus the heating coil may not be sufficiently cooled and may be deteriorated rapidly.